Method for screening using electrostatic adhesion

ABSTRACT

The present invention relates generally to a new apparatus and method for screening using electrostatic adhesion. More particularly, the invention encompasses an apparatus that uses an electrostatic charge during the screening process for a semiconductor substrate. Basically, a backing layer is adhered to a green ceramic sheet using an electrostatic charge, while the green ceramic sheet is processed.

FIELD OF THE INVENTION

The present invention relates generally to a new apparatus and methodfor screening using electrostatic adhesion. More particularly, theinvention encompasses an apparatus that uses an electrostatic chargeduring the screening process for a semiconductor substrate. Basically, abacking layer is adhered to a green ceramic sheet using an electrostaticcharge, while the green ceramic sheet is processed.

BACKGROUND OF THE INVENTION

Semiconductor devices are becoming smaller and more dense with theevolution of new technology. Therefore, there is a pressure for thisindustry to constantly make improvements and innovation in thistechnology. The semiconductor manufacturers are therefore constantlybeing challenged to improve the quality of their products by identifyingand eliminating defects and to make improvements where necessary.Whereas significant improvements are being made to eliminate systematicdefects by reducing process variability. Process improvements alone arenot sufficient to eliminate all the random defects which effect bothyield and reliability. Historically, inspection techniques have beenemployed to improve product failure rates to acceptable levels byculling out many of these random defects.

Similarly, in the fabrication of ceramic substrates, such as, multilayerceramic (MLC) substrates, the formation of vias is critical to theelectrical performance of the final substrate. Vias can be formed by anumber of methods. The most popular method of forming vias in ceramicgreen sheets is by punching or drilling holes through the individualgreen sheets and filling these holes with a metallurgical paste that iselectrically conductive.

These individual green sheets are then laminated by methods well knownin the art and the green sheet laminates are then processed through afurnace. After sintering of the laminated green sheets and thesubsequent formation of a ceramic substrate, the vias containing themetal form an electrical path between the various layers on thesubstrate.

U.S. Pat. No. 3,892,614 (Levy), discloses a electrostatic laminatingapparatus and method in which two or more sheets to be pinned togetherare passed in close proximity or in contact with each other through anelectric field and the resulting electrostatic charge pins these twosheets together.

U.S. Pat. No. 4,457,972 (Griffith), teaches adhesion of layers usinghigh energy bombardment. Basically, films are firmly bonded to insulatorsubstrates by irradiating the interface with high energy ions.

U.S. Pat. No. 5,167,997 (Chamberlain), discloses the formation ofprotected conductive foil assemblage using static electrical forces. Aplastic film is removably joined with an electrically conductive foil.The absence of air between the film and the foil is produced byoppositely statically electrically charging the film and the foil suchthat they are forced together sufficiently to squeeze the air out fromtherebetween.

IBM Technical Disclosure Bulletin, "Backing Material For MLC Screening",Vol. 24, No. 10, page 5119 (March 1982), teaches that backing materialis typically employed in the MLC (Multi-Layer Ceramic) screeningprocesses. It also teaches that the backing material prevents paste atthe via holes from contacting the screening nest. And, that the backingmaterial should also exhibit porosity or perforations for vacuum holddown of the green sheet and it should have non-porous edges for vacuumhandling. Additionally, it discloses that the backing material utilizedfor this prior art screening processes should exhibit low static chargesusceptibility, is smooth and planar, and is compatible with thescreening paste used, and also exhibits dimensional stability.

In most of the prior art processes pits and/or depressions are commonlyformed during the screening process. The formation of pits ordepressions in the green sheet screening process is basically associatedwith volumetric changes during the drying of the paste. During thedrying cycle the solvents are removed and the paste volume decreases.Additionally, pits are also formed if the interface between the pasteand the backing film is somehow degraded. However, if the paste adheresto the backing film and it easily separates upon drying then pitting isminimized. This fixed interface forces the volumetric shrinkage to occurat the opposite surface of the green sheet which is the actual bottomsurface of the top layer of the completed substrate.

The present invention relates to a new method and apparatus for thefabrication of green sheets for MLC applications which contain a flush,pit-free via surface. This is accomplished by applying a temporaryelectrostatic charge to the ceramic green sheet and a polymeric orinsulating backing film. The electrostatic charge results in anelectrostatic adhesion of the two layers or material. This adhesiveforce prevents the movement of the green sheet relative to the backingfilm, thus maintaining a good interface between the two films/sheets.After the screening process has been performed the electrostatic chargeis removed and subsequently the backing layer is peeled off, leavingbehind a green ceramic sheet with pit-free vias.

PURPOSES AND SUMMARY OF THE INVENTION

The invention is a novel method and an apparatus for using anelectrostatic charge to provide adhesion between a green sheet and abacking layer during the screening process.

Therefore, one purpose of this invention is to provide an apparatus anda method that will provide electrostatic adhesion between the greensheet and the backing layer or the bi-layer during the screeningprocess.

Another purpose of this invention is to provide for a pit free viaformation in a green sheet.

Still another purpose of this invention is to have an economical way ofmaking pit free vias in a green ceramic sheets.

Yet another purpose of this invention is to provide a way to remove thestatic charge from the bi-layer after the screening process.

Therefore, in one aspect this invention comprises a process forscreening a green sheet using an electrostatic charge, comprising thesteps of:

(a) placing said green sheet with at least one hole on at least onebacking sheet, such that said backing sheet entirely covers said atleast one hole and forms a bi-layer,

(b) electrostatically charging said bi-layer, such that said backingsheet statically adheres to at least a portion of said green sheet,

(c) screening said bi-layer with at least one metallic paste, such thatsaid paste fills said at least one hole in said green sheet,

(d) placing said bi-layer in an antistatic environment after saidmetallic paste has dried and removing said charge from said bi-layer,and

(e) removing said backing layer and thereby completing the screening ofa green sheet using an electrostatic charge.

In another aspect this invention comprises a bi-layer green sheetstructure, comprising,

(a) at least one green sheet with at least one via hole and at least onebacking layer,

(b) said at least one green sheet and said at least one backing layerbeing electrostatically charged, such that the electrostatic charge onsaid green sheet is opposite of the electrostatic charge on said backinglayer, and

(c) metallic paste in said at least one via.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The drawings are for illustration purposes only and arenot drawn to scale. Furthermore, like numbers represent like features inthe drawings. The invention itself, however, both as to organization andmethod of operation, may best be understood by reference to the detaileddescription which follows taken in conjunction with the accompanyingdrawings in which:

FIG. 1, illustrates a prior art green ceramic sheet having vias whichhave pits and depressions.

FIG. 2, illustrates a side view of the preferred embodiment of thisinvention during the electrostatic charging process.

FIG. 3, illustrates a side view of the invention of FIG. 2, after ascreening mask has been placed over the green sheet.

FIG. 4, shows the bi-layer having the electrostatic charge after viaholes in the green sheet have been filled.

FIG. 5, is a side view of the inventive bi-layer during theelectrostatic discharge process.

FIG. 6, is a side view of the green ceramic sheet after the backinglayer has been removed.

DETAILED DESCRIPTION OF THE INVENTION

The prior art process of filling the via holes in most cases involvesextruding a metal filled paste into the via holes either by using astencil type mask or a mesh type mask. Since the metallurgical pastecontains solvents which are required to achieve the necessaryrheological behavior for screening, the paste must be dried after thevias have been filled in order to remove the above mentioned solvents.

During the drying of the metallurgical pastes, the solvents are removedand significant shrinkage of the paste can occur. This shrinkage isassociated with the volumetric change resulting from capillary dryingstresses during the solvent removal process. If allowed to occur freely,the shrinkage of the paste typically results in a structure thatcontains depressions and pits at the top and bottom surfaces of thevia/green sheet.

FIG. 1, shows a cross-section of a prior art ceramic substrate 10,having ceramic sheet 12, that has gone through a sintering cycle and hasvias 14, which contain pits 16, on the upper surface 13, and depressions18, on the bottom surface 17, of the ceramic sheet 12. The surfaceindicated by numeral 15, is a smooth via surface 15, which is presentlynot available all the time by the prior art methods and processes.

These depressions 18, and pits 16, can create significant problems iftheir size is sufficient to cause poor electrical and/or mechanicalconnection between the layers 12, during lamination or if the pits 16,are present at the surface after lamination.

Furthermore, open pits 16, at the surface of the vias 14, can trapfluids and debris after sintering, which could result in significantprocessing problems of the fired or sintered substrate. Ideally, aflush, dense surface of the paste in the via is required to achievemaximum electrical performance of MLC substrates, as shown by thesurface 15.

Several techniques can be used to reduce or eliminate capillary stressand shrinkage associated with solvent removal during drying. Thesetechniques typically involve altering the surface energies of the systemby using surfactants or super critical drying processes. However, thesetechniques are typically not suitable to the processing of green sheetsfor MLC applications. As such, a technique which can control thelocalized drying of the paste at the free surfaces such that pits ordepressions are eliminated, without affecting the green sheet, would bevery beneficial.

However, one solution to this problem could be to simply apply anadhesive (not shown) to a backing film, in order to aid in themaintenance between the green sheet 12, and the backing film. While thisapproach is technically feasible, removal of the adhered backing film,after screening could result in stretching or distortion of the greensheet 12, or in significant damage to the green sheet 12, and the paste,which is similar to the one shown in FIG. 1. This of course negates theusefulness of this approach.

Thus there is a need to invent a technique to reduce or eliminate theformation of drying pits or depressions, that may form in a ceramicgreen sheet.

FIG. 2, illustrates the preferred embodiment of this invention. Astandard ceramic green sheet 12, having holes or openings 11, is placedonto a backing film or layer 20, to form a bi-layer 25. It is highlydesirable that the backing film 20, is insulative and non-conductive.The material for the backing film could be made from a group comprisinglatex, poly(alkenes), polyester, polymeric material, polyimide, rubber,to name a few. An insulative polymer by the tradename MELINEX, couldalso be used as a backing layer. However, it should be understood thatany backing layer that is used, must be non-conductive or insulative inorder to maintain the electrostatic charge.

Using charge bars 22 and 26, electrostatic charge is provided to theceramic green sheet 12, and the backing film 20, to form a temporaryattractive force between the green sheet 12, and the backing film 20,that is used for the screening process. The charge bars 22 and 26, canbe AC or DC charge bars. However, at least one charge bar 22 or 26,needs to be a DC source, while the other bar can be an AC or DC chargebar.

Actual polarity of the charging is not critical as long as sufficientcharge is applied to temporarily adhere the backing film 20, to thegreen sheet 12. As shown in FIG. 2, the positive charge 27, has beenapplied to the surface 13, of the green sheet 12, while negative charge23, has been applied to the surface 21, of the backing film 20.

Any movement between the green sheet 12, and the backing film 20, duringthe actual screening operation could result in a degradation to themetallurgical paste/backing film interface.

Furthermore, maintaining a good interface between the green sheet 12,and the backing film 20, is very critical during the drying of thepaste, because once the interface is compromised, the paste is free toshrink away from the backing film 20, resulting in drying pits. Thus,the better the interface between the backing film 20, and the greensheet/paste 12, the lower the level of drying pits.

The amount of charge 23 and 27, can be tailored to the process bycontrolling the applied voltages to the charging bars 22 and 26,respectively. Voltages as high as 20,000 to 30,000 volts can and havebeen used. Since the green sheet 12, and the backing film 20, areinsulators, the charge is stable over a long period of time and isdifficult to remove during normal processing and handling since theoverall bi-layer 25, (green sheet and backing film) is essentiallyneutral in charge.

Once the bi-layer 25, has been charged, the adhesion between the greensheet 12, and backing film 20, is such that the movement of the tworelative to each other is essentially eliminated during normal handling.

The bi-layer 25, containing the charged green sheet 12, and the chargedbacking film 20, is now ready for the next processing step, as shown inFIG. 3. Screening of the green sheet 12, is performed using standardprocesses, such as, placing a stencil type screening mask 30, over thecharged surface 13, of the green sheet 12, and screening metallic paste14, through an opening 31, in the mask 30, into the opening or via holes11, of the green sheet 12, as shown in FIG. 3. Care should be taken thatthe paste 14, does not overflow out of the via opening 11.

The screening paste 14, is typically selected from a group comprisingcopper, gold, molybdenum, nickel, platinum, silver, tungsten, or alloysthereof, to name a few.

After the screening process, the bi-layer 25, containing themetallurgical paste 14, is dried using standard drying methods andconditions.

The electrostatic charge 27 and 23, between the green sheet 12, and thebacking film 20, remains during the drying operation, thus maintaininglittle or no movement of the green sheet 12, on the backing film 20.This reduces or eliminates separation of the metallic paste 14, from thebacking film 20, during the drying of the paste 14. Sufficient adhesionof the metallic paste 14, to the backing film 20, typically will occurto prevent separation between the two, as long as the interface is notmechanically compromised by sheet movement.

As stated earlier and as shown in FIG. 4, that due to the electrostaticcharge the bi-layer 25, comprising the green sheet 12, and backing film20, will maintain very close adherence even after the screening and thedrying of the paste in the via process. It should be emphasized that thebi-layers 25, still contains significant amounts of electrostatic chargeat this point. In fact, removal of the backing film 20, without chargeneutralization can result in damage to the via 14, due to pulling out ofdried paste 14.

To eliminate this problem, as shown in FIG. 5, the bi-layer 25, is firstdischarged by passing the bi-layer 25, through two AC antistatic bars35, or by using an antistatic ion gun 35. Both of these devices flowlarge amounts of positive and negative ions over both surfaces 13 and21, of the bi-layer 25. This effectively neutralizes the charge 23 and27, on the bi-layer 25.

As illustrated in FIG. 6, that once the charge 23 and 27, has beenneutralized, the backing film 20, can be simply and easily removed whichresults in a green ceramic layer 50, having a ceramic layer 12, with atleast one via 14. Wherein the via 14, has a flush, pit and depressionfree via surface 15, at the upper surface 13, and has a smooth, flush,pit and depression free surface 45, at the lower surface 17. However, insome cases it may have some depressions on the free surface. It shouldbe noted that the smooth surface 45, basically is due to the presence ofthe backing layer 20, that was electrostatically adhered to the surface17, of the green ceramic sheet 12.

It should also be appreciated that the use of electrostatic attractionto hold the ceramic layer and the backing layer is completelyreversible, and no chemical residues are left at the end of the process.

The backing film should be selected to be sufficiently resistant toscreening and/or drying temperature and pressure conditions, and alsoremain adhered to the ceramic green sheet. The backing film should alsobe removable from the green ceramic sheet after screening withoutcausing any damage to the screened green sheet.

For most applications it is preferred that the green sheet is a ceramicgreen sheet. However, the green sheet could be selected from a groupcomprising aluminum nitride, aluminum oxide, glass-ceramic, to name afew.

The electrostatic charge provides a very good adhesive force andprevents any movement of the green sheet relative to the backing film,thus maintaining a very good interface between the green sheet and thebacking layer. The elimination of the movement at the interface of thebacking material and the green sheet is very critical for the productionof the pit-free vias. Therefore, the electrostatic charge that isapplied to the bi-layer must be such that there is no movement betweenthe bi-layers.

This invention also enables the placement of controllable amount ofstatic charge on both sides of the bi-layer. This results in acontrolled static adhesion between the two materials of the bi-layer andwhich is also reversible.

It should be understood that the electrostatic charge should be highenough to provide good electrostatic adhesion between the backing layerand the green sheet. A low electrostatic charge will not be strongenough to hold the backing layer and the green sheet together throughoutthe screening process. Therefore, the preferred electrostatic charge isat least 1,000 volts, however, for most applications the electrostaticcharge should be between about 1,000 volts and about 100,000 volts,preferably between about 10,000 volts and about 50,000 volts, and morepreferably between about 20,000 volts and about 40,000 volts.

It should be appreciated that some of the green ceramic sheets doacquire an electrostatic charge during normal processing. However, whileusing this invention all green ceramic sheets should pass through anantistatic environment prior to charging of the bi-layer, so that theonly charge present on these green sheets are the electrostatic chargethat has been applied using the process of this invention.

It should also be noted that this invention provides a chemical freeadhesion during the screening process, as no adhesive chemical residuegets transferred from any of the layers. And, also the adhesion iscompletely reversible, i.e., it does not matter what the polarity ofcharge is applied to any of the layers, as long as the second charge isthe opposite of the first charge.

EXAMPLE

The following example is intended to further illustrate the inventionand is not intended to limit the scope of the invention in any manner.

EXAMPLE 1

To demonstrate the effectiveness of this invention, a controlledexperiment was run using electrostatic adhesion and comparing this touncharged green sheets.

Two control samples were processed. The first control samples wereprocessed by the prior art methods and it was found that on an averagethe first control samples had a 71 percent pitting level. On the otherhand when the second set of controlled samples were processed with themethod of this invention, that is, the backing layer and the ceramiclayer were electrostatically charged, the pitting level was only 3percent.

These values were typical and apply only to the controlled experimentsrun for the evaluations. Actual ranges will depend on numerousprocessing variables and as such, these data should be used only forcomparison purposes.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

What is claimed is:
 1. A process for screening a green sheet using an electrostatic charge, comprising the steps of:(a) placing said green sheet with at least one hole on at least one backing sheet, such that said backing sheet entirely covers said at least one hole and forms a bi-layer, (b) electrostatically charging said bi-layer, such that said backing sheet statically adheres to at least a portion of said green sheet, (c) screening said bi-layer with at least one metallic paste, such that said paste fills said at least one hole in said green sheet, (d) placing said bi-layer in an antistatic environment after said metallic paste has dried and removing said charge from said bi-layer, and (e) removing said backing layer and thereby completing the screening of a green sheet using an electrostatic charge.
 2. The process of claim 1, wherein said green sheet is selected from a group consisting of aluminum nitride, aluminum oxide, ceramic or glass-ceramic.
 3. The process of claim 1, wherein said backing material is selected from a group consisting of latex, poly(alkenes), polyester, polymeric material, polyimide or rubber.
 4. The process of claim 1, wherein said electrostatic charge is between about 1,000 volts and about 100,000 volts, preferably between about 10,000 volts and about 50,000 volts, and more preferably between about 20,000 volts and about 40,000 volts.
 5. The process of claim 1, wherein said antistatic environment is provided by at least two antistatic bars.
 6. The process of claim 1, wherein said screening paste is selected from a group consisting of copper, gold, molybdenum, nickel, platinum, silver, tungsten, or alloys thereof.
 7. The process of claim 1, wherein the surface of said dried metallic paste is flushed smooth with the surface of said green sheet. 